Tennessee Wastewater Operator Practice Exam

The settleability test on MLSS indicates settling performance in which type of unit?

1. Primary clarifier

2. Secondary clarifier

3. Aeration tank

4. Sludge drying bed

The correct answer is: Secondary clarifier

The settleability test on mixed liquor suspended solids (MLSS) is primarily indicative of the settling performance in a secondary clarifier. This test evaluates how well the activated sludge can drop out of suspension and form a clear supernatant liquid above the settled sludge once it is allowed to settle. In the context of secondary clarifiers, the settling performance is crucial as it directly affects the efficiency of the biological treatment process. Proper settling ensures that enough solids are removed from the effluent, allowing for effective further treatment or discharge. If the MLSS settles well, it indicates good floc formation and density, which are essential for optimizing the operation of the secondary clarification process. In contrast, primary clarifiers focus on removing solids from incoming wastewater before biological treatment, while aeration tanks are concerned with the oxygenation process for microorganisms to degrade organic matter. Sludge drying beds are used for dewatering settled sludge, not for assessing the performance of biomass settling. Thus, the settleability test is most relevant to the secondary clarifier's function in the overall wastewater treatment system.